1. Field of the Invention
The present invention relates to a laterally diffused metal-oxide-semiconductor device, and more particularly to a laterally diffused metal-oxide-semiconductor device which can improve capacitance characteristics with the optimized on-resistance.
2. Description of the Prior Art
A laterally diffused metal-oxide-semiconductor (LDMOS) device is a common semiconductor power device. Due to the horizontal structure of the laterally diffused metal-oxide-semiconductor device, it is easily formed and integrated with the conventional semiconductor techniques so as to reduce the manufacturing cost. Since the laterally diffused metal-oxide-semiconductor device has a higher breakdown voltage and a higher output power, it is widely applied to the electronic devices, such as a power converter, a power amplifier, a switch, and a rectifier.
Please refer to FIG. 1, which is a schematic cross-sectional view illustrating a laterally diffused metal-oxide-semiconductor device in the prior art. As shown in FIG. 1, a conventional laterally diffused metal-oxide-semiconductor device is usually disposed in a p-type semiconductor substrate 10, and the p-type semiconductor substrate 10 includes an n-type drift region 11, a p-type body region 12, an n-type source region 13, and an n-type drain region 14. The p-type body region 12 is disposed in the n-type drift region 11, and the n-type source region 13 is disposed in the p-type body region 12. The n-type drain region 14 is disposed in the n-type drift region 11 and at a side of the n-type source region 13. Furthermore, a source electrode 15 is disposed on the n-type source region 13, and a drain electrode 16 is disposed on the n-type drain region 14. A gate electrode 17 is disposed between the source electrode 15 and the drain electrode 16. In addition, a field oxide layer 18 is disposed on n-type drift region 11 and between the gate electrode 17 and the drain electrode 16.
As shown in FIG. 1, the voltage-withstand ability of the laterally diffused metal-oxide-semiconductor device in the prior art is usually improved by increasing the length of the drift region or additionally disposing a field oxide layer. However, increasing the horizontal extension length of the device not only decreases the integration degree of the device, but also increases the on-resistance of the device. On the other hand, a trench semiconductor device with the vertical structure is developed in the prior art to improve the laterally diffused metal-oxide-semiconductor device, such as reducing the on-resistance. But the trench semiconductor device has a high gate-to-drain capacitance (Cgd). Accordingly, when the trench semiconductor device is applied to the switching device, this high gate-to-drain capacitance will increase the transition time of the voltage and influences the power loss. As a result, a semiconductor device which can improve both of the capacitance characteristic and the on-resistance is required in the industry.